Data plan required for foreign phones used in US?

I searched here and on google and did not find a definite answer.
I'm looking for a way to circumvent AT&T's forced data plans (there's wifi EVERYWHERE!!). As far as I know, AT&T towers can read the IMEI number of the phone you are using. That tells them what phone it is, where you got it, and what version it is. So when AT&T sees that you have your sim card in a smartphone, they send you a text saying, "we slapped a data plan onto your account". However, I think that maybe AT&T can only enforce the data plans on phones sold in the US.
My dad got a Nokia N85 (this is a smartphone, right?) from China and has been using it for at least two year now and he's not getting a forced add-on data plan. It could be because he was using the phone without a data plan before the smartphone+data combo that he's not being charged. We just renewed our contract, but my dad did not get a text about adding data charges, or AT&T will charge him without notification at the end of the month.
Does anyone else have a foreign smartphone in the US on AT&T? Any data charges?
-Nos

Ok. I don't know exactly how things work in the US. But I'm 99% certain when I tell you that you will be charged for data usage by your carrier one way or another. Even if it's with a data plan, or you just pay for every MB you use.

It's extremely unlikely that your father is not getting charged for data. I'd check his bill very carefully, even if he doesn't have a data plan, he can still be charged by usage. If he isn't getting charged, check what his APN settings are and post them here